1. Field of the Invention
The present invention relates to an information filtering apparatus, for example, of the type used with multiplexing units coupled to a sources of a plurality of data signals, such as a photodiode array. The present invention also relates to a method of filtering the plurality of data signals received.
2. Prior Art
In the field of Acousto-Optic (AO) processors, it is known to optically process an electronic signal, having a bandwidth B, using a Bragg cell disposed between a so collimated light source and a lens, the lens being arranged to perform a Fourier transform on light propagating from the Bragg cell. Generally speaking, the spatial distribution of light at the focus of the lens (hereinafter referred to as the ‘Fourier distribution’) is the Fourier transform of the spatial distribution of the light at the aperture let of the lens. The Fourier distribution at the focus of the lens is detected by an array of photodetectors. The electronic signal is therefore converted into N frequency channels, where N is the number of photodetectors in the array of photodetectors, each frequency channel having a nominal bandwidth of B/N. Additionally, the Bragg cell can be modulated in order to produce a varying Fourier distribution at the array of photodetectors.
The array of photodetectors is typically packaged as an integrated circuit comprising a line of, for example, 1000 photo-lithographically produced photodetector elements. Thus, if the bandwidth B of the electronic signal is 1 GHz, the signal generated by each photodetector element has a bandwidth of 1 MHz (1 GHz/1000). The number of photodetector elements is usuall possessed by the integrated circuit, thereby necessitating the provision of a multiplexer in the same package as the array of photodetectors. The multiplexer samples each photodetector element and transfers the voltage/charge on the photodetector element to an output buffer coupled to other electronic circuitry via the pins.
Currently available multiplexers will take around 100 μs to scan all the photodetector elements (1000), which implies an output bandwidth of 5 kHz for each photodetector element; this is much less than the bandwidth of 1 MHz of the signals generated by the photodetector elements. Therefore, it can be seen that a great deal of information is received by each photodetector element is being lost. Furthermore, in some applications information relating to only a small number of frequencies in the bandwidth of the electronic signal may be required. Thus, processing information relating to frequencies, other than those required, is a waste of multiplexer resources.